/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.bangcommunity.bbframe.common.tuple;

import com.bangcommunity.bbframe.common.utils.lang.ObjectUtils;

import java.io.Serializable;
import java.util.Map;

/**
 * <p>
 * A pair consisting of two elements.
 * </p>
 * 
 * <p>
 * This class is an abstract implementation defining the basic API. It refers to the elements as
 * 'left' and 'right'. It also implements the {@code Map.Entry} interface where the key is 'left'
 * and the value is 'right'.
 * </p>
 * 
 * <p>
 * Subclass implementations may be mutable or immutable. However, there is no restriction on the
 * type of the stored objects that may be stored. If mutable objects are stored in the pair, then
 * the pair itself effectively becomes mutable.
 * </p>
 *
 * @param <L>
 *            the left element type
 * @param <R>
 *            the right element type
 *
 * @since Lang 3.0
 * @version $Id: Pair.java 1557584 2014-01-12 18:26:49Z britter $
 */
public abstract class Pair<L, R> implements Map.Entry<L, R>, Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 4954918890077093841L;

    /**
     * <p>
     * Obtains an immutable pair of from two objects inferring the generic types.
     * </p>
     * 
     * <p>
     * This factory allows the pair to be created using inference to obtain the generic types.
     * </p>
     * 
     * @param <L>
     *            the left element type
     * @param <R>
     *            the right element type
     * @param left
     *            the left element, may be null
     * @param right
     *            the right element, may be null
     * @return a pair formed from the two parameters, not null
     */
    public static <L, R> Pair<L, R> of(final L left, final R right) {
        return new ImmutablePair<L, R>(left, right);
    }

    // -----------------------------------------------------------------------
    /**
     * <p>
     * Gets the left element from this pair.
     * </p>
     * 
     * <p>
     * When treated as a key-value pair, this is the key.
     * </p>
     * 
     * @return the left element, may be null
     */
    public abstract L getLeft();

    /**
     * <p>
     * Gets the right element from this pair.
     * </p>
     * 
     * <p>
     * When treated as a key-value pair, this is the value.
     * </p>
     * 
     * @return the right element, may be null
     */
    public abstract R getRight();

    /**
     * <p>
     * Gets the key from this pair.
     * </p>
     * 
     * <p>
     * This method implements the {@code Map.Entry} interface returning the left element as the key.
     * </p>
     * 
     * @return the left element as the key, may be null
     */
    @Override
    public final L getKey() {
        return getLeft();
    }

    /**
     * <p>
     * Gets the value from this pair.
     * </p>
     * 
     * <p>
     * This method implements the {@code Map.Entry} interface returning the right element as the
     * value.
     * </p>
     * 
     * @return the right element as the value, may be null
     */
    @Override
    public R getValue() {
        return getRight();
    }

    // -----------------------------------------------------------------------

    /**
     * <p>
     * Compares this pair to another based on the two elements.
     * </p>
     * 
     * @param obj
     *            the object to compare to, null returns false
     * @return true if the elements of the pair are equal
     */
    @SuppressWarnings("deprecation") // ObjectUtils.equals(Object, Object) has been deprecated in
                                     // 3.2
    @Override
    public boolean equals(final Object obj) {
        if (obj == this) {
            return true;
        }
        if (obj instanceof Map.Entry<?, ?>) {
            final Map.Entry<?, ?> other = (Map.Entry<?, ?>) obj;
            return ObjectUtils.nullSafeEquals(getKey(), other.getKey())
                    && ObjectUtils.nullSafeEquals(getValue(), other.getValue());
        }
        return false;
    }

    /**
     * <p>
     * Returns a suitable hash code. The hash code follows the definition in {@code Map.Entry}.
     * </p>
     * 
     * @return the hash code
     */
    @Override
    public int hashCode() {
        // see Map.Entry API specification
        return (getKey() == null ? 0 : getKey().hashCode()) ^ (getValue() == null ? 0 : getValue().hashCode());
    }

    /**
     * <p>
     * Returns a String representation of this pair using the format {@code ($left,$right)}.
     * </p>
     * 
     * @return a string describing this object, not null
     */
    @Override
    public String toString() {
        return new StringBuilder().append('(').append(getLeft()).append(',').append(getRight()).append(')').toString();
    }

    /**
     * <p>
     * Formats the receiver using the given format.
     * </p>
     * 
     * <p>
     * This uses {@link java.util.Formattable} to perform the formatting. Two variables may be used
     * to embed the left and right elements. Use {@code %1$s} for the left element (key) and
     * {@code %2$s} for the right element (value). The default format used by {@code toString()} is
     * {@code (%1$s,%2$s)}.
     * </p>
     * 
     * @param format
     *            the format string, optionally containing {@code %1$s} and {@code %2$s}, not null
     * @return the formatted string, not null
     */
    public String toString(final String format) {
        return String.format(format, getLeft(), getRight());
    }

}
